In order to denitrate combustion exhaust gas, ammonia (NH3) is used as a reducing agent for decomposing nitrogen oxides (NOx) into nitrogen (N2) and water vapor (H2O). In this denitration reaction, a DeNOx (denitration) catalyst is used in order to improve the reaction rate and reaction efficiency. For improving the decomposition rate of NOx, it is only necessary to improve a NH3/NOx molar ratio when NH3 is added. When the NH3/NOx molar ratio is increased excessively, however, unreacted ammonia may be mixed into an exhaust gas. The resulting exhaust gas, when released in the atmosphere, may possibly cause secondary pollution or the like due to leaked ammonia.
Under such a situation, an exhaust gas purifying catalyst having a catalyst layer for the oxidation of unreacted ammonia is known as is disclosed in JP3436567 B.
Such an exhaust gas purifying catalyst comprises a DeNOx catalyst layer as thick as approximately 0.5 mm formed using a DeNOx catalyst slurry. This layer is essential to assure denitration reaction. Thickening of the DeNOx catalyst layer however increases a pressure loss so that the operation efficiency of a plant deteriorates. Another reason for thickening the DeNOx catalyst layer is that the catalyst carrier layer cannot be used as an exhaust gas purifying catalyst.